1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to an improved apparatus and method for measuring a food temperature in a microwave oven, which makes it possible to precisely measure a temperature of the food placed in a microwave oven by providing thereto a simple circuit composition in order to carry out the measurement without regard to kinds of food, cooking, heating device or temperature variation therearound.
2. Description of the Prior Art
As shown in FIG. 1, a general microwave oven includes: a turntable 30 for receiving food 20 thereon in a heating chamber 10; a driving motor 40 for rotating the turntable 30; a magnetron 50 and a heater 60 for respectively serving to heat the food 20 in the form of microwave energy or heat; a temperature detecting circuit 70 for detecting infrared rays being emitted from the food 20 and measuring a temperature of the food 20; and a microcomputer 80 for controlling the operation of the magnetron 50 and the heater 60 in accordance with the temperature measured in the temperature detecting circuit 70.
As further shown in FIG. 2, the temperature detecting circuit 70 includes: a thermopile sensor 11 outputting a voltage Va corresponding to the infrared rays emitted from the food 20 and a voltage Vb corresponding to an ambient temperature; and a temperature amplifier 12 for compensating for a voltage Vd decreased or increased in conjunction with the ambient temperature under the output voltage Va of the thermopile sensor 11.
The temperature amplifier 12 includes: an amplifier 12a for amplifying the output voltage Va of the thermopile sensor 11; a temperature compensator 12b for amplifying the output voltage Vb of the thermopile sensor 11 and outputting the compensated voltage Vd, an adder 12c for adding an output voltage Vc of the amplifier 12a and the compensating voltage Vd of the temperature compensator 13; and an amplifier 12d for amplifying an output valve of the adder 12c and outputting a temperature detecting voltage V0. Here, Vc denotes a ground voltage in common.
The operation of the thusly constituted conventional microwave oven and the temperature measuring circuit 70 will now be described with reference to FIGS. 1 and 2.
First, the food 20 for being cooked is placed on the turntable 30 by a user, and a cooking type is keyed in by using a menu setting board, so that the microcomputer 80 recognizes the input menu and accordingly serves to drive the turntable driving motor 40, and the magnetron 50 or the heater 60.
The food 20 placed on the turntable 30 is rotated in conformation with the driving of the turntable driving motor 40, and the rotating food 20 starts heated by microwave energy or heat in accordance with the driving of the magnetron 50 or the heater 60.
At this time, the thermopile sensor 11 in the temperature measuring circuit 70 outputs the voltage Va corresponding to the infrared rays reflected from the food 20 and received through an incidence window A.
Here, the output voltage Va in the thermopile sensor 11 becomes variable depending upon the ambient temperature, so that the effect resulting from the ambient temperature should be compensated for by using a temperature compensating device which may be a thermistor or a diode disposed inside or around the thermopile sensor 11. When the temperature compensating device serves as a thermistor, the output valve of the temperature compensating device becomes a resistance value, and when the temperature compensating device serves as a diode, the output valve of the temperature compensating device becomes a voltage value.
With reference to FIG. 2, a diode is employed as a temperature compensating device and disposed inside the thermopile sensor 11.
The thermopile sensor 11 outputs the voltage Va generated due to the infrared rays emitted from the food 20, and the voltage Vb measured in an internal temperature compensating device (not shown).
The amplifier 12a of the temperature amplifier 12 amplifies the output voltage Va of the thermopile sensor 11. The temperature compensator 12b amplifies the output voltage Vb of the temperature compensating device (not shown). The adder 14 adds the output voltage Vc of the amplifier 12a and the output voltage Vb of the temperature compensator 12b.
At this time, the output of the adder 12c becomes a voltage in accordance with the surface temperature of the food 20 which is obtained by removing the effect of the ambient temperature of the thermopile sensor 11 from the output voltage Va of the thermopile sensor 11. The amplifier 12d amplifies the output of the adder 12c and outputs the amplified resultant valve to the microcomputer 80 which detects the temperature of the food 20 in accordance with the output voltage V0 of the temperature amplifier 12, and in accordance with the detected temperature, the time required for thawing or cooking the food 20 is calculated, during which time the magnetron 50 or the heater 60 is driven to thereby carry out a thawing or cooking function.
Referring to FIG. 3 illustrating another example of the temperature amplifier 12, the thermistor 14 serving as the temperature compensating device is disposed adjacent to the thermopile sensor 11.
The temperature amplifier 13 includes: an amplifier 13a a non-inverting terminal of which is connected to the output voltage Va of the thermopile sensor 11, and an inverting terminal of which is connected through a resistance R1 to a reference voltage Vref and through a resistance R2 to an output terminal thereof; an amplifier 13b a non-inverting terminal of which is connected to the reference voltage Vref, and an inverting terminal of which is connected through a resistance R3 and the resistance Rth of the thermistor 14 to a ground voltage and at the same time through the resistance R4 to an output terminal thereof; an adder 13c for adding the respective voltage values outputted from the amplifiers 13a, 13b; and an amplifier 13d a non-inverting terminal of which is connected to the output terminal of the adder 13c, and an inverting terminal of which is connected through a resistance R7 to the reference voltage Vref and through a resistance R8 to an output terminal thereof.
The operation of the thusly constituted temperature measuring circuit 70 will now be described with reference to FIG. 3.
The thermopile sensor 11 receives the infrared rays emitted from the food surface via the incidence window A, and outputs voltage Va corresponding to the received infrared rays. The amplifier 13a compares the output voltage Va of the thermopile sensor 11 with the reference voltage Vref, amplifies and outputs to the adder 13c the resultant value, in accordance with Equation 1. ##EQU1##
The amplifier 13b of the temperature compensator 12b compares the voltage measured adjacent to the thermopile sensor 11 with the reference voltage Vref, amplifies the compared value and outputs the compensating voltage Vd to the adder 13c, according to Equation 2. ##EQU2##
Here, when the respective values of the resistance R5 and the resistance R6 are set identically to each other, the output value of the adder 12c becomes (Vc+Vd)/2, and the amplifier 13d compares the output valve of the adder 12c with the reference voltage Vref and amplifies and outputs to the microcomputer 80 the compared value, in accordance with Equation 3. ##EQU3##
That is, referring to Equation 3, when the ambient temperature of the thermopile sensor 11 is raised by 1C.degree., the output voltage Vc of the amplifier 13a is decreased by 10 mV, and when the resistance pair R3, R4 are set in order for voltage Vd to become increased by 10 mV, the effect by the ambient temperature of the thermopile sensor 11 can be excluded.
Therefore, the microcomputer 80 detects the temperature of the food 20 in accordance with the output voltage V0 of the temperature amplifier 12, and in accordance with the detected temperature, the time for thawing or cooking the food 20 is obtained, during which time the magnetron 50 or the heater 60 is driven for thereby carrying out the thawing or cooking operation.
In general, in the case in which a magnetron is employed as a heating member, the respective temperatures of the food 20 and the heating chamber 10 do not generally exceed 120.degree. C., and when no food is placed on the turntable 30, the temperature of the turntable 30 exceeds 150.degree. C.
When the heater 60 is employed to heat the food 20, the temperature of the heating chamber 10 comes to exceed 200.degree. C., and the temperature variation for thawing or cooking the food 20 ranges from -10.degree. C. to 200.degree. C.
Here, in case of thawing the food 20, the microcomputer 80 is required to measure the food temperature by 1.degree. C. unit.
At this time, although cost-effective types are applied to the amplifiers 13a, 13b, 13d in consideration of their prices, the output voltage of the amplifiers do not exceed 3.3 V at the worst, and it is difficult to lower the minimal voltage than 0.5 V due to a technical limit in design.
The microcomputer 80 is operated under 5 V to thereby process an 8-bit data set, the voltage disassembling capacity is referred to as 5 V/(2).sup.2 =5/256=0.0195 V.
As a result, the output voltage of the amplifier 12d becomes 2.8 V, and when the microcomputer 80 measures the temperature by 1.degree. C. unit, the temperature measurement is confined to a scope ranging from -10.degree. C. to 134.degree. C. (2.8 V/0.0195 V=144) instead of from -10.degree. C. to 200.degree. C.
Also, the temperature measurement of the microcomputer 80 is confined to a scope ranging from -10.degree. C. to 134.degree. C., so that when the magnetron is selected as a heating member, there is a possibility in which an over heating or fire may occur. Further, when employing the heater, the temperature variation of the food ranges from -10.degree. C. to 200.degree. C., for thereby disabling temperature measurement of the food.
Conclusively, in the conventional microwave oven, the temperature measurement range of food is confined by the limited measurement scope of the microcomputer 80, and therefore the conventional microwave oven does not appropriately respond to the temperature measurement range which is variable depending upon whether the food is placed on the turntable, and upon the kinds of cooking and heating member.
Further, the conventional microwave oven separately provides the temperature compensator 12b and the fourth amplifier for excluding influence resulting from the ambient temperature, and the temperature measurement circuit 70 is formed of a plurality of parts, for thereby deteriorating price competitiveness.